Why uranium enrichment was slow/near impossible in the beginning of Manhattan project but few years later it became trivial?

Question

So I’ve watched Oppenheimer and a recurring theme in the movie was how tedious task it was to get fission material (demonstrated as filling a large fish bowl by marbles). All they manage to collect in two year period was just enough for two bombs. but fast forward few years US have a complete arsenal of bombs to flat the earth. I understand they must’ve innovated a method and the exact method maybe classified, what i’m interested in is knowing what was the obstacle(s) for this and rough idea of how they might’ve overcome it.

In: 450

Answer 1

They had to develop the methods and tools do do it. I don’t work with nuclear stuff, but I am a design engineer. It is hard and time consuming in general, but add in completely new technologies and it just gets exponentially harder. Look at it this way, designing, making and testing the production is a long slow process. But once it’s built and running, it can make a lot more stuff.